Allow excluding folders from SourceKit-LSP

[award999]

Description

A long standing user request of the vscode-swift extension has been the ability to exclude certain Swift packages: swiftlang/vscode-swift#636

With this PR we can exclude configured folders from initializing in the vscode extension swiftlang/vscode-swift#1693. The outstanding issue is that if an excluded file is open, SourceKit-LSP still loads and builds the package.

It would be nice if the documentService/languageService logic in the LSP could be amended to be able to exclude configured files which can be specified in the initialize request or in the .sourcekit-lsp/config.json file

[ahoppen]

I disagree that this should be a SourceKit-LSP option. If the editor opens a document in SourceKit-LSP it is communicating a clear intention that it wishes to receive semantic functionality. If the user does not wish to receive semantic functionality for a document, I think it’s the editor’s job to ensure that it does not get opened in the LSP server.